Infrared imaging of ultrasonically excited subsurface defects in materials

ABSTRACT

A thermal imaging system for detecting cracks and defects in a component. An ultrasonic transducer is coupled to the specimen through a malleable coupler. Ultrasonic energy from the transducer causes the defects to heat up, which is detected by a thermal camera. The ultrasonic energy is in the form of a pulse where the frequency of the ultrasonic signal is substantially constant within the pulse. A control unit is employed to provide timing and control for the operation of the ultrasonic transducer and the camera.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and method for thedetection of defects in a material and, more particularly, to atechnique for coupling ultrasonic energy into a material to heat cracksand other defects that may exist in the material, and then thermallyimaging the material to identify the defects by the heat radiatingtherefrom.

2. Discussion of the Related Art

Maintaining the structural integrity of certain components andstructures is very important in many areas because of safety concernsand the like. Loss of structural integrity is typically caused bymaterial defects, such as cracks, delaminations, disbonds, corrosion,inclusions, voids and the like, that may exist in the component orstructure. For example, it is very important in the aviation industrythat reliable techniques are available to examine the structuralintegrity of the aircraft skin and structural components of the aircraftto ensure that the aircraft does not suffer from structural failure whenin flight. The structural integrity of turbine blades and rotors, andvehicle cylinder heads is also important in those industries. Therefore,techniques have been developed for the non-invasive and non-destructiveanalysis of different structural components and materials in variousindustries.

One known technique for non-invasive and non-destructive testing formaterial defects includes treating the structural component with a dyepenetrant so that the dye enters any crack or defects that may bepresent in the material. The component is then cleaned, and thestructure is treated with a powder that causes the dye remaining in thecracks to wick into the powder. An ultraviolet (UV) light source is usedto inspect the material to observe locations on the component thatfluoresces as a result of the dye. This technique has the disadvantage,however, that it is highly inspector intensive and dependent because theperson inspecting for the fluorescence must be skilled. Additionally,the dye does not typically penetrate tightly closed cracks or cracksthat are not on the surface.

A second known technique for inspecting a component for defects employsan electromagnetic coil to induce eddy currents in the component. Thecoil is moved around on the component, and the eddy current patternchanges at a crack or other defect. The complex impedance in the coilchanges as the eddy current changes, which can be observed on anoscilloscope. This technique has the drawback that it is also veryoperator intensive, and also extremely slow and tedious.

Another known technique employs thermal imaging of the component toidentify the defects. Typically, a heat source, such as a flash lamp ora heat gun, is used to direct a planar pulse of heat to the surface ofthe component. The material of the component absorbs the heat, and emitsreflections in the infrared wavelengths. Certain types of defects willcause the surface temperature to cool at a different rate over thedefects than for the surrounding areas. A thermal or infrared imagingcamera is used to image the component and detect the resulting surfacetemperature variation. Although this technique has been successful fordetecting disbonds and corrosion, it is ordinarily not successful fordetecting vertical cracks in the material, that is, those cracks thatare perpendicular to the surface. This is because a fatigue crack lookslike a knife edge to the planar heat pulse, and therefore no, orminimal, reflections occur from the crack making the cracks hard orimpossible to see in the thermal image.

Thermal imaging for detecting defects in a material has been extended tosystems that employ ultrasonic excitation of the material to generatethe heat. The article Rantala, J. et al, “Lock-in Thermography withMechanical Loss Angle Heating at Ultrasonic Frequencies,” QuantitativeInfrared Thermography, Eurotherm Series 50, Edizioni ETS, Pisa 1997, pg389-393 discloses such a technique. In this technique, ultrasonicexcitation is used to cause the crack or defect to “light up” as aresult of the ultrasonic field. Particularly, the ultrasonic waves causethe opposing edges of the crack to rub together causing the crack areato heat up. Because the undamaged part of the component is onlyminimally heated by the ultrasonic waves, the resulting thermal imagesof the material show the cracks as bright areas against a darkbackground field.

The transducer used in the ultrasonic thermal imaging technique referredto above makes a mechanical contact with the component being analyzed.However, it is difficult to couple high power ultrasonic energy intosome materials, particularly in the case of metals. Significantimprovements in this technique can be achieved by improving the couplingbetween the ultrasonic transducer and the component.

Additionally, the known ultrasonic thermal imaging technique employscomplex signal processing, particularly vector lock-in, synchronousimaging. Vector lock-in imaging uses a periodically modulated ultrasonicsource and includes a processing technique that synchronously averagessuccessive image frames producing an in-phase image and a quadratureimage both based on the periodicity of the source. This results inimages that are synchronous with the periodicity and eliminatesunsynchronous noise from the image. The periodicity of the image canalso be induced by an external stimulus, such as a modulated laser beam,heat lamps, etc. The processor receives the frames of video images andstores them synchronously with the induced periodicity, and thenaverages the stored frames with subsequently received frames to removethe noise. U.S. Pat. No. 4,878,116 issued Oct. 31, 1989 issued to Thomaset al discloses this type of vector lock-in imaging.

U.S. Pat. No. 5,287,183 issued to Thomas et al Feb. 15, 1994 discloses asynchronous imaging technique that is a modification of the vectorlock-in imaging disclosed in the '116 patent. Particularly, the imagingtechnique disclosed in the '183 patent extends the vector lock-insynchronous imaging technique to include a “box car” technique variationwhere the source is pulsed, and the images are synchronously averaged atvarious delay times following each pulse. The box car techniquemultiplies the video signal by zero except in several narrow timewindows, referred to as gates, which are at a fixed time delay from theinitiation of each ultrasonic pulse. The effect of these gates is toacquire several images corresponding to the states of component beingimaged at the predetermined fixed delay times after the pulses. Thesedifferent delay times are analogous to the different phases, representedby the sine and cosine functions of the periodic signal in the lock-intechnique. During the acquisition of the gated images, the imagescorresponding to different delay times are combined arithmetically bypixel-by-pixel subtraction to suppress non-synchronous backgroundeffects.

The ultrasonic excitation thermal imaging technique has been successfulfor detecting cracks. However, this technique can be improved upon todetect smaller cracks, as well as tightly closed cracks, with muchgreater sensitivity. It is therefore an object of the present inventionto provide such a defect detection technique.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a techniqueis disclosed for infrared or thermal imaging of ultrasonically excitedsubsurface defects in a material. An ultrasonic source is connected to aspecimen being inspected through a coupler that transmits the ultrasonicwaves into the material with minimum attenuation. The ultrasonic sourceemits a single ultrasonic pulse having a constant frequency amplitudefor a predetermined period of time. A suitable thermal imaging camera isused to image the specimen when it is being excited by the ultrasonicsource. A control unit is used to control the operation of theultrasonic source and the camera for timing purposes. Although vectorlock-in, or box car integration, synchronous imaging techniques can beemployed for reducing noise in the images, such signal processingtechniques are not required in the present invention.

During initiation of the detection sequence, the control unit instructsthe camera to begin taking sequential images of the specimen. Next, thecontrol unit instructs the transducer to emit a pulse of ultrasonicenergy at a predetermined frequency for a predetermined time period. Asequence of images are generated that show cracks and other defects inthe material as light areas (higher temperature) against a dark (lowertemperature) background. The images can be displayed on a monitor, and astorage device can be provided to store the sequence of images to bereviewed at a later time.

Additional objects, advantages and features of the present inventionwill become apparent from the following description and the appendedclaims when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an imaging system according to theinvention;

FIG. 2 is a broken-away, side view of the transducer, specimen andcamera of the imaging system shown in FIG. 1;

FIG. 3 is a graph with power on the vertical axis and time on thehorizontal axis showing the ultrasonic signal used in the known thermalimaging techniques that employ vector lock-in synchronous imaging;

FIG. 4 is a graph with power on the vertical axis and time on thehorizontal axis showing the pulsed ultrasonic signal used in the thermalimaging technique of the present invention;

FIGS. 5(a)-5(d) show consecutive images at predetermined time intervalsof an open crack in a specimen that has been ultrasonically excited andimaged by an imaging system of the present invention;

FIG. 6 is an image generated by the imaging system of the invention,showing a closed crack excited by ultrasonic energy;

FIG. 7 is an image generated by the imaging system of the presentinvention, showing a delamination or disbond excited by the ultrasonicenergy; and

FIG. 8 is a perspective view of a person holding an ultrasonictransducer against an aircraft component, and using the imaging systemof the present invention to detect cracks in the component.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments directed to anultrasonic and thermal imaging system is merely exemplary in nature, andis in no way intended to limit the invention or its applications oruses.

FIG. 1 is a block diagram of an imaging system 10, according to anembodiment of the present invention. The imaging system 10 is being usedto detect defects, such as cracks, corrosion, delaminations, disbonds,etc., in a specimen 12. The specimen 12 is intended to represent anystructural component or material, such as an aircraft skin, that mayinclude these types of defects. It is stressed that specimen 12 does notneed to be metal, but can be other materials, such as ceramics,composites, etc. The system 10 includes an ultrasonic transducer 14having a piezoelectric element that generates ultrasonic energy within acertain ultrasonic frequency band. The transducer 14 can be anytransducer suitable for the purposes described herein, such as theBranson 900 MA ultrasonic transducer. In one embodiment, the ultrasonictransducer 14 generates a pulse of ultrasonic energy having asubstantially constant amplitude at a frequency of about 20 kHz for aperiod of time of about ½ of a second and at a power level of about 1kW. However, as will be appreciated by those skilled in the art, otherultrasonic frequencies or sonic frequencies, power levels and pulsedurations can be used within the scope of the present invention.

The ultrasonic energy from the transducer 14 is coupled to the specimen12 through a coupler 16. The coupler 16 is in mechanical contact with anend 18 of the transducer 14 and a front side 20 of the specimen 12. FIG.2 is a broken-away, side view showing the transducer 14 in contact withthe coupler 16 and the specimen 12. A support structure 26 is used tohelp maintain the transducer 14 in contact with the coupler 16. In oneembodiment, the coupler 16 is a thin piece of a soft metal, such ascopper, to effectively couple the ultrasonic energy into the specimen12. Of course, other couplers consistent with the discussion herein canbe used. The coupler 16 can be any suitable piece of material that istypically softer than the end 18 of the transducer 14, and is malleableto be deformed against the end 18 of the transducer 14 and prevent thetransducer 14 from bouncing from or walking along the specimen 12. Inone embodiment, the coupler 16 couples about 30 to 40 percent of theultrasonic energy from the transducer 14 into the specimen 12. It isnoted, however, that the coupler 16 may not be needed in certainapplications, such as testing for defects in a composite.

A thermal imaging camera 22 is provided and spaced from a back side 24of the specimen 12, and generates images of the side 24 of the specimen12 in association with ultrasonic excitations of the specimen 12. Thecamera 22 can be spaced from the specimen 12 any suitable distance toprovide images of as much of the specimen as desired in a single image.In other embodiments, the ultrasonic energy from transducer 14 and theimage generated by the camera 22 can be provided at the same side of thespecimen 12. The thermal camera 22 can be any camera suitable for thepurposes described herein, such as the Galileo camera available fromRaytheon. In one embodiment, the camera 22 senses infrared emissions inthe 3-5 micron wavelength range, and generates images at 100 frames persecond. The camera 22 includes a focal plane array having 256×256 InSbpixels to generate the resolution desirable. In one embodiment, the side24 of the specimen 12 is painted black to provide better contrast forinfrared imaging.

A controller 30 provides timing between the transducer 14 and the camera22. The controller 30 can be any computer suitable for the purposesdescribed herein. When the detection process is initiated, thecontroller 30 causes the camera 22 to begin taking sequential images ofthe specimen 12 at a predetermined rate. Once the sequence of imagesbegins, the controller 30 sends a signal to an amplifier 32 that causesthe amplifier 32 to send a pulse to the transducer 14 to generate thepulsed ultrasonic signal. The ultrasonic energy is in the form of asimple pulse at the frequency being used. It is not necessary to employany type of vector lock-in or synchronous imaging techniques between thepulse of energy and the imaging, as is currently done in the prior art.However, such signal processing techniques can be used to further reducenoise. It is stressed that the frequencies and pulse time periods beingdescribed herein are by way of non-limiting examples, in that differentfrequencies, pulse times, input power, etc. will vary from system tosystem and specimen being tested. After the end of the pulse, thecontroller 30 instructs the camera 22 to stop taking images. The imagesgenerated by the camera 22 are sent to a monitor 34 that displays theimages of the side 24 of the specimen 12. The images can then be sent toa storage device 36 to be viewed at another location if desirable.

The ultrasonic energy applied to the specimen 12 causes faces of thedefects and cracks in the specimen 12 to rub against each other andcreate heat. This heat appears as bright spots in the images generatedby the camera 22. Therefore, the system is very good at identifying verysmall tightly closed cracks. For those cracks that may be open, wherethe faces of the crack do not touch, the heating is generated at thestress concentration point at the crack tip. This point appears as abright spot on the images indicating the end or tip of an open crack.The ultrasonic energy is effective to heat the crack or defect in thespecimen 12 no matter what the orientation of the crack is relative tothe energy pulse. The camera 22 takes an image of the surface 24 of thespecimen 12, providing a visual indication of any crack in the specimen12 no matter what the position of the crack within the thickness of thespecimen 12.

The present invention provides improvements over the known ultrasonicand thermal imaging techniques because the ultrasonic pulses used toheat the cracks and defects are simple pulses having a substantiallyconstant amplitude, and do not need to employ sinusoidal signalmodulation as used in vector lock-in synchronous imaging. To illustratethis point, FIG. 3 shows a graph with power on the vertical axis andtime on the horizontal axis depicting the waveform of the ultrasonicsignal used in vector lock-in imaging. The ultrasonic signal isgenerated at a predetermined frequency, and modulated with a lowfrequency sinusoidal modulating wave that provides amplitude modulationat a predetermined modulation period. The ultrasonic frequency signalrises and falls in amplitude with the low frequency modulation wave.Typically, the ultrasonic excitation is performed over several seconds.The image generated by this imaging technique is not the actual image ofthe particular component being imaged, but is a difference imagegenerated by the subtraction process of the synchronous imaging. A moredetailed discussion of this type of vector lock-in synchronous imagingto reduce noise in these types of systems is discussed in the '116patent.

FIG. 4 is a graph with power on the vertical axis and time on thehorizontal axis showing the pulses used to provide the ultrasonicexcitation in the present invention. The ultrasonic frequency signalwithin each pulse has substantially the same amplitude, and is notmodulated by a lower frequency sinusoidal waveform. The images generatedby the camera 22 are real images, and not difference images of the typegenerated in the vector lock-in synchronous imaging technique. Thisprovides a significant improvement in image quality and controlsimplicity. Although one pulse is ordinarily sufficient, more than onepulse can be employed, separated in time by a predetermined time period,for signal averaging purposes to reduce noise. The technique of “boxcar” integration can be used as discussed in the '183 patent. In thistechnique, a gate is used in each time window to identify an image foreach pulse, where the gate is at a certain fixed time delay from thebeginning of the pulse. During the acquisition of the gated images, theimages corresponding to different delay times are combinedarithmetically to suppress non-synchronous background effects.

FIGS. 5(a)-5(d) show four sequential images 38 of an open fatigue crack40 in a metal specimen 42. FIG. 5(a) shows the images 38 of the specimen42 prior to the ultrasonic energy being applied. FIG. 5(b) shows theimage 38 of the specimen 42 14 ms after the ultrasonic energy isapplied. As is apparent, a light (higher temperature) spot 44 (sketchedas a dark region) appears at the closed end of the crack 40, where themechanical agitation causes the heating. FIGS. 5(c) and 5(d) showsubsequent images 38 at time equal to 64 ms and time equal to 114 ms,respectively. The light spot 44 on the specimen 42 increasesdramatically over this sequence, clearly indicating the location of thecrack 40.

FIG. 6 shows an image 48 of a closed crack 50 in a specimen 52 afterbeing energized by the ultrasonic pulse. In this embodiment, because thecrack 50 is closed, the entire length of the crack 50 generates heatcreating a light spot 54 along the entire length of the crack 50 andproviding an indication of a closed crack. Because the ultrasonic energyis so effective in causing the closed crack 50 to heat up significantlyrelative to the background, very short closed cracks, for example on theorder of ⅔ mm, are readily ascertainable in the image 48.

FIG. 7 shows an image 66 of a specimen 68. In this image, a light spot70 is shown, and is intended to represent the type of image generatedfrom the thermal energy that is created by ultrasonically exciting adelamination or disbond. The thermal imaging technique of the presentinvention is particularly useful in identifying “kissing” disbonds.

FIG. 8 is a perspective view of an operator 56 holding a hand-heldtransducer 58 against a specimen 60, such as an aircraft fuselage. Athermal imaging camera 62 is directed towards the specimen 60 at alocation that is separate from the point of contact of the transducer58. FIG. 8 illustrates that the system according to the invention can beused in the field for testing such components.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A thermal imaging system for detecting defects ina component, said system comprising: a sound source; a thermal imagingcamera directed towards the component and generating thermal images ofthe component; and a controller electrically connected to the soundsource and the camera, said controller causing the sound source to emitat least one pulse of a sound signal at a predetermined frequency andfor a predetermined duration, and causing the camera to generatesequential images of the component, wherein the frequency of the soundsignal has a substantially constant amplitude, and wherein vibrationalenergy from the sound source causes the defects in the component to heatup and be visible in the images generated by the camera.
 2. The systemaccording to claim 1 wherein the at least one pulse is a plurality ofpulses separated by a predetermined time, and wherein the controlleraverages images taken within a predetermined window in each pulse, wherethe window is provided at the same delay time in each pulse from whenthe pulse is initiated.
 3. The system according to claim 1 wherein theat least one pulse is a series of pulses, and wherein the system employsa box car integration technique to provide signal averaging of theimages.
 4. The system according to claim 1 wherein the sound sourceemits the pulse for a duration of one-half of a second or less.
 5. Thesystem according to claim 1 further comprising a coupler positionedbetween and in contact with the component and the sound source.
 6. Thesystem according to claim 5 wherein the coupler is malleable and issofter than an end of the sound source in contact with the coupler. 7.The system according to claim 5 wherein the coupler is a thin piece ofmetal.
 8. The system according to claim 7 wherein the coupler is acopper member.
 9. The system according to claim 1 further comprising amonitor responsive to the images from the camera, said monitordisplaying the images.
 10. The system according to claim 1 wherein thecamera is directed towards the component at a location that is separatefrom the location where the sound source contacts the component.
 11. Adefect detection system for detecting defects in a structure, saidsystem comprising: a sound source for directing at least one pulse of asound signal into the structure for a predetermined period of time, saidsignal having a predetermined frequency and a substantially constantamplitude; a camera directed towards the structure and generating imagesof the structure when the source emits the sound signal; and acontroller connected to the source and the camera to provide timingsignals therebetween, said controller being responsive to the imagesfrom the camera.
 12. The system according to claim 11 wherein the atleast one pulse is a plurality of pulses separated by a predeterminedtime, and wherein the system averages images taken within apredetermined window in each pulse, where the window is provided at thesame delay time in each pulse from when the pulse is initiated.
 13. Thesystem according to claim 11 wherein the at least one pulse is a seriesof pulses, and wherein the system employs a box car integrationtechnique to provide signal averaging of the images.
 14. The systemaccording to claim 11 wherein the sound source emits the pulse for aduration of one-half of a second or less.
 15. The system according toclaim 11 further comprising a coupler positioned in contact with thestructure and the source, said coupler being a malleable member.
 16. Thesystem according to claim 15 wherein the coupler is a thin piece ofmetal.
 17. The system according to claim 11 wherein the controllercauses the camera to take sequential images of the structure before,during and after when the sound source emits the sound signal.
 18. Thesystem according to claim 11 wherein the camera is directed towards thestructure at a location that is separate from a location where thesource contacts the structure.
 19. A method of detecting defects in astructure, said method comprising the steps of: emitting at least onepulse of a sound signal into the structure to heat the defects, saidsignal having a predetermined frequency and a substantially constantamplitude; and generating a sequence of thermal images of the structureprior to, during and after the emission of the sound signal.
 20. Themethod according to claim 19 further comprising the steps of emitting aplurality of pulses of the sound signal, and providing signal averagingof images generated in a window in each separate pulse where the windowis provided at the same delay time in each pulse from when the pulse isinitiated.
 21. The method according to claim 19 further comprising thestep of processing the images using a box car synchronous imagingtechnique.
 22. The method according to claim 19 wherein the step ofemitting a pulse of the sound signal includes emitting the pulse for aduration of one-half of a second or less.
 23. The method according toclaim 19 further comprising the step of providing a coupler in contactwith the structure and in contact with a sound source that generates thesound signal.
 24. The method according to claim 23 wherein the step ofproviding a coupler includes providing a coupler that is malleable andconforms to an end of the sound source in contact with the structure.25. The system according to claim 1 wherein the sound source is anultrasonic transducer and the at least one pulse is an ultrasonic pulse.26. The system according to claim 11 wherein the sound source is anultrasonic source and the sound signal is an ultrasonic signal.
 27. Themethod according to claim 19 wherein the step of emitting at least onepulse includes emitting at least one pulse of ultrasonic energy.
 28. Adefect detection system for detecting defects in a structure, saidsystem comprising: a sound source; a coupler positioned between and incontact with the structure and the sound source; a thermal imagingcamera directed towards the structure, and generating thermal images ofthe structure; and a controller electrically coupled to the sound sourceand the camera, said controller causing the sound source to emit a soundsignal at a predetermined frequency for a predetermined duration, thesignal having a substantially constant amplituide, and causing thecamera to generate sequential images of the structure, wherein the soundsignal from the sound source causes the defects in the structure to heatup and be visible in the images generated by the camera. The limitationthat the sound signal has a substantially constant amplitude wasnecessary to overcome any application of the prior art.
 29. The systemaccording to claim 28 wherein the coupler is malleable and is softerthan an end of the sound source.
 30. The system according to claim 29wherein the coupler is a thin piece of metal.
 31. The system accordingto claim 30 wherein the coupler is a copper member.